There has conventionally been known a thermal printer as example of a printing apparatus that controls a thermal head subjected to application of a main pulse or a sub pulse.
In a conventional thermal printer, there has been the conventional problem as follows: generally, a thermal printer carries out printing by selectively heating a plurality of heater elements arranged at least in one row on its thermal head. Heating of selected heater elements is carried out by applying a driving pulse to each selected heater element for predetermined period of time. In addition to application of a main pulse for heating the selected heater element, a sub pulse shorter in pulse application period of time than a main pulse is applied at the beginning of printing so as to compensate energy shortage and to prevent a weak print-out. Thus, in the case of thermal printing, the thermal head accumulates heat as printing continues further. In a case where patterns are continuously printed with high density at high heating temperature, heat accumulation can possibly cause printing faults such as characters printed bolder than intended, unclear print-out, etc.
The above description will be explained in detail hereinafter. As shown in FIG. 10, there is assumed a case to successively print out plural dots P1, P2, P3 . . . from the beginning of printing. In the above conventional printing control, there is arranged one (first) pulse application period F1 made up of a series of a sub-pulse application time S, a main-pulse application time M1 and a non-heating time C1 for printing out a dot P1. Subsequent to the one (first) pulse application period F1, there is arranged other (second) pulse application period F2 made up of a series of a main-pulse application time M2 and a non-heating time C2 for printing out a dot P2. Subsequent to the other (second) pulse application period F2, there is further arranged other (third) pulse application period F3 made up of a series of a main-pulse application time M3 and a non-heating time C3 for printing out a dot P3. In similar with the above, subsequent to the other (third) pulse application period F3, there are further arranged other (fourth and after) pulse application periods each of which is made up of a series of a main-pulse application time and a non-heating time for printing out a dot subsequent to a previous dot. Incidentally, in the drawings of the present application, a pulse application is indicated as low active and electric power supplied during pulse application is constant.
In the one (first) pulse application period F1, for energy shortage compensation, there is secured the sub-pulse application time S. That is, a sub pulse with short energization time is applied so that the dot P1 is printed out without a weak print-out.
In the pulse application periods F1, F2, F3 . . . , there are respectively secured main-pulse application times M1, M2, M3 . . . . For printing out each of the dots P1, P2, P3 . . . , a main pulse with long energization time is applied in each main-pulse application time.
In this connection, it is preferable to correct length of the sub-pulse application time S and that of each of the main-pulse application times M1, M2, M3, . . . , depending on temperature of heater elements on the thermal head. However, it is difficult to directly measure temperature of heater elements on the thermal head. Therefore, length of an application time is corrected based on temperature detected by a thermistor disposed at a location set off from heater elements on the thermal head. Accordingly, length of each of non-heating times C1, C2, C3 . . . is corrected, as well.
Temperature detected with the thermistor is lower than actual temperature of heater elements on the thermal head. As printing continues further, difference between measured temperature and actual temperature of heater elements grows. The thermal head accumulates heat.
In addition, as printing speed is made faster, the thermal head is more likely to accumulate heat. This is because temperature at heater elements on the thermal head does not go down sufficiently to reach predetermined temperature. Regarding other (second and after) pulse application periods F to follow the one (first) pulse application period F1, FIG. 11 comparatively shows configuration of a pulse application period F at printing speed of 10 mm/sec. and that of a pulse application period F at printing speed of 30 mm/sec. As shown in FIG. 11, proportion of a main-pulse application time M to a pulse application period F of the latter one is larger than that of the former one. In other words, proportion of a non-heating time C to an application period F of the latter one is smaller than that of the former one. Accordingly, as shown in FIG. 12, temperature at heater elements on the thermal head goes down to reach 50 degrees Celsius or lower when printed at printing speed of 10 mm/sec., whereas temperature thereof cannot go down to reach 50 degrees Celsius or lower when printed at printing speed of 30 mm/sec. or faster.
The above such heat accumulation in the thermal head becomes more significant as temperature detected with the thermistor is higher.
As explained in detail, as continuous printing goes further, as printing speed is made faster, or as temperature detected with a thermistor is higher, a thermal heard is more likely to accumulate heat. Heat accumulation could possibly cause printing faults such as characters printed bolder than intended, unclear print-out, etc.
Even if length of application time is corrected from the one (first) application period F1 simply in consideration of continuous printing, printing speed and heat accumulation of a thermal head, the correction of pulse application time from the one (first) application period F1 can adversely cause pulse-application energy shortage in a case where sufficient heat accumulation has not been secured before the start of the one (first) application period F1. That is, even if a sub-pulse application time S is secured in the one (first) application period F1, pulse-application energy shortage can possibly occur and make weak print-out of the dot P inevitable in the case where sufficient heat accumulation has not been secured before the start of the one (first) application period F1.